Commercial passenger-carrying aircraft are typically powered by high bypass turbofan gas turbine engines for propelling the aircraft at subsonic speeds. Aircraft are configured for carrying different numbers of passengers over different ranges and, therefore, require engines having different maximum thrust capability. Furthermore, the engine is specifically configured to match the thrust levels required by the aircraft to reduce specific fuel consumption (SFC) and thereby reduce total fuel burned.
Accordingly, an engine designed for one aircraft configuration having a specific maximum thrust capability and a corresponding low SFC operating range is typically not usable for a different aircraft application requiring a different level of maximum thrust and different low SFC operating range. However, in order to reduce development and operational costs, aircraft engines are typically developed in families sharing as many common components as possible while reducing unique components for each design application for covering a range of maximum thrust capability and corresponding low SFC operational ranges.
For example, an engine family may use a common core engine including a high pressure compressor, combustor, and high pressure turbine for providing combustion gases to a low pressure turbine for powering the fan. The fan size, or outer diameter, is a primary factor in the maximum thrust capability of the engine. Larger fan diameter allows increased propulsion thrust from the engine, but also increases size and weight of the engine which adversely affect fuel burn. Uninstalled SFC may be improved for subsonic turbofan engines as fan pressure ratio is reduced. However, as fan pressure ratio is reduced the airflow through the fan must increase to retain the required maximum thrust from the engine. This can best be appreciated by the fundamental relationship between specific thrust, which is the pounds of thrust per pound of airflow, and the fan pressure ratio. Specific thrust is reduced as the fan pressure ratio is reduced. Therefore, to improve SFC by reducing fan pressure ratio, an increase in fan airflow is required which, in turn, requires a larger diameter fan which increases engine weight and increases nacelle scrubbing and interference drags. These effects diminish the uninstalled advantage of a low fan-pressure ratio engine to the point that when the engine is installed in the aircraft, the overall efficiency of operation is diminishingly reduced.
Furthermore, a plot of SFC versus thrust for a given engine includes a conventionally known throttle hook wherein the plot is generally U-shaped with SFC having a minimum value at an intermediate thrust level of the engine. The width of the bottom, or bucket, of the throttle hook, which is the thrust range over which the SFC remains substantially constant, is an important parameter of the engine. For example, a relatively wide bucket allows a given engine to be used for a wider range of different aircraft configurations which operate at a substantial amount of time in the bucket region for reducing fuel consumption. Such applications may include smaller and lighter aircraft, or the end of a long flight where most of the aircraft fuel has been burned off and, therefore, requires less thrust for cruise operation.
Both a wider throttle hook for a given engine family, and a larger range of maximum thrust therefrom increase the number of different aircraft configurations which may effectively use the engine family.